The bottom surface of a capsule, such as the Orion capsule, becomes extremely hot upon reentry into the Earth's atmosphere. To protect the capsule, the bottom surface is covered with a thermally resistant material known as a thermal protection system (TPS).
By its very nature, when entering the Earth's atmosphere, the TPS material absorbs vibration, and therefore, absorbs sound. This imposes added difficulties during inspection of the heat shield blocks that are bonded to the composite (e.g., blocks bonded to the bottom surface of the capsule). Additional difficulties include the fact that the TPS material is relatively low in density, has mechanical damping, is thermally insulated, and is inhomogeneous. Because the TPS material absorbs heat, thermal technologies cannot be used to inspect the bonded interface between the TPS material and the composite. Also, because inspection may be performed from only the outer surface of the TPS material, conventional inspection technologies, such as x-ray technology, cannot be used to inspect the bonded interface since the x-ray source and detector must be placed on opposite sides of the TPS.
Furthermore, it is imperative that the technique detects kissing unbonds. A kissing unbond is defined as two surfaces that are in contact, but are not actually bonded. Electromagnetic technologies, such as x-ray and terahertz technologies, may be used to detect changes in the bondline, such as air gaps. However, electromagnetic technologies cannot differentiate between bond and unbond conditions when an air gap is missing. Consequently, electromagnetic technologies cannot detect kissing unbonds.
Thus, inspection of the bonded interface may be complex, and an improved approach is desirable.